#include <iostream>
using namespace std;
int main()
{
    cout << "Hello world!" << endl;
    //判断是否是回文字符串
    string str;
    cin >> str; //输入字符串
    for (int i = 0; i < str.size() / 2; i++)
    {
        if (str[i] != str[str.size() - i - 1])
        {
            cout << "不是回文字符串" << endl;
            return 0;
        }
    }
    cout << "是回文字符串" << endl;
    return 0;
}
